It has long been recognized that an electronic generator of musical sounds cannot, by itself, duplicate the acoustic effect of, say, a string orchestra or a pipe organ from which nominally identical notes are perceived by the ear of a listener with slight time-varying phase differences. Thus, U.S. Pat. No. 3,257,495 (Williams) describes a system in which two oscillators of different frequencies in the sub-audio range, specifically of 6 and 13 Hz, modulate an audio signal in the output of an electronic organ to create a complex vibrato effect. U.S. Pat. No. 3,833,752 (Van der Kooij) teaches the use of three signal channels connected in parallel to an electronic-organ output, these channels including respective delay lines of the charge-transfer type controlled by gating-pulse trains from generators responsive to a modulating signal synthesized from two sub-audio-frequency oscillations, specifically of about 1 and 5 Hz, which pass through separate phase shifters designed to introduce predetermined phase differences between the three modulating signals. The insertion of individual modulators in such parallel signal channels, in lieu of controllable delay lines, is known from U.S. Pat. No. 3,979,991 (Kawamoto).